The present invention relates to multistage dry vacuum pumps such as Roots type multistage pumps, claw type pumps, and combined Roots-and-claw type pumps.
Such multistage dry vacuum pumps are made up of a plurality of compression stages connected in series.
FIGS. 1 and 2 show a multistage Roots type dry pump of the prior art. FIG. 2 is a perspective view in longitudinal section showing the stator of such a Roots pump. In the stator 1, between a gas inlet 2 and a gas outlet 3, there can be seen five successive compression chambers respectively referenced 4, 5, 6, 7 and 8. Adjacent chambers are separated by respective transverse walls 9, 10, 11, and 12 each pierced by two holes such as the holes 13 and 14 in the transverse wall 12 for passing the shafts of two parallel rotors, not shown, that are mechanically coupled together, and that carry compression lobes of the Roots or claw type. Adjacent chambers are interconnected via a gas flow duct such as the gas flow duct 15 connecting the delivery outlet of the first compression chamber 4 to the suction inlet of the second compression chamber 5.
The rotor lobes that penetrate into the compression chambers 4-8 are of a diameter that is greater than that of the rotor shafts that pass through the holes 13 and 14. It is therefore not possible to engage an entire rotor axially in the stator 1 by mere axial displacement. Nor is it possible to envisage machining a one-piece stator 1 in such a manner as to make the cavities constituting the compression chambers 4-8.
To make both machining and assembly possible, and also to provide good sealing, the stators of known dry vacuum pumps are generally built up as an axial assembly of a plurality of stator elements, respectively referenced 16, 17, 18, 19 and 20, which are assembled to one another via their respective front end walls such as the front end wall 21 of stator element 16, with interposed between the walls respective sealing rings 22, 23, 24, 25 and 26 that become compressed axially so as to isolate each compression chamber 4-8 from the outside atmosphere.
Such a structure for a dry pump of the Roots or claw type requires each stator element 16-20 to be machined separately, and then it requires an assembly operation to be performed that is lengthy and difficult, consisting in fitting both rotor shafts in a support frame, in adjusting the positions of the lobes in the last compression chamber 8, positioning the last stator element 20 together with the sealing ring 26, fitting the lobes for the last compression chamber but one 7, bringing the last stator element but one 19 together with the sealing ring 25 into position, and so on to the first stator element 16. Given that clearance between the rotor lobes and the walls of the stator is very small in order to seal each compression stage of the vacuum pump, it will be understood that such assembly is particularly lengthy and difficult to implement, and it is generally accepted that several hours of labor are required to perform this operation on a five-stage dry vacuum pump.
Another problem, in such known multistage dry vacuum pumps, is the difficulty of aligning the stator elements with one another, given that errors are liable to accumulate between the first stator element 16 and the last stator element 20, thus making it difficult to control the clearance between the rotors and the stator in mass production.
Documents EP 0 476 631 A and JP 03 145594 A describe vacuum pump structures having a stator made up of two half-shells that are assembled together radially with a longitudinal assembly surface generally parallel to the axes of the rotors, the stator being closed in leaktight manner as its ends by two fitted endpieces that are engaged axially. Those documents do not mention the advantage of such a stator structure in the form of two half-shells, and they do not describe means for providing sealing between the stator and the rotor.
The difficulty lies in the need both to provide peripheral radial sealing in the longitudinal assembly surface between the two half-shells so as to prevent gases passing between the outside atmosphere and the internal cavities of the pump, while simultaneously providing axial sealing at the ends between the half-shells and the fitted endpieces.
In traditional manner, it might be imagined that axial sealing at the ends could be provided by sealing rings of the kind shown in FIGS. 1 and 2 for the prior art pump, and that radial peripheral sealing could be provided by longitudinal gaskets compressed between the two half-shells. Unfortunately, that solution presents a major drawback stemming from the fact that leakage lines exist between the longitudinal gasket providing peripheral radial sealing and the O-rings providing axial sealing at the ends. Sealing therefore is unsatisfactory.
The problem posed by the present invention is that of designing a new multistage dry vacuum pump structure that makes it possible to reduce significantly the number of parts to be assembled during assembly, while facilitating assembly and enabling it to be performed more quickly, and while also providing sealing that is satisfactory between the internal cavities of the vacuum pump and the outside atmosphere so as to avoid any risks of the pumped gases being polluted by the outside atmosphere, and any risks of the outside atmosphere being polluted by the pumped gases.
The solution of the invention consists in providing a continuous one-piece sealing gasket which provides both types of sealing simultaneously in a stator structure in the form of two half-shells.
Thus, to achieve these objects, and others, the invention provides a multistage dry vacuum pump made up of a plurality of compression stages placed in series, the pump having at least one rotor mounted to rotate in a stator that is closed in leaktight manner at its ends by two fitted end pieces; furthermore:
the stator is made by radially assembling together two half-shells on a longitudinal assembly surface, each compression stage thus being contained in two corresponding portions of each of the half-shells, the two half-shells, once assembled together, containing all of the compression stages;
a continuous one-piece gasket serves to provide both peripheral radial sealing in the longitudinal assembly surface of the half-shells, and axial end sealing between the half-shells and the fitted end pieces so as to isolate the compression stages from the outside atmosphere.
In an advantageous embodiment, the gasket comprises two annular end portions that are generally parallel to each other and that are interconnected by two longitudinally-extending portions that are generally perpendicular thereto.
With a sealing gasket of this structure, in the assembled state, the longitudinally-extending portions of the gasket are compressed laterally between the two half-shells in the longitudinal assembly surface, while the two annular end portions are compressed between the two half-shells acting together on the one hand and the respective fitted end pieces on the other hand.
To provide good compression of the first annular end portion of the gasket, it is advantageous to provide a first fitted end piece that has an axial nose shaped to occupy a corresponding axial recess in the first end of the stator body as made up by the two assembled-together half-shells. In this way, the first annular end portion of the gasket is compressed radially by the two half-shells against the axial nose.
Preferably, the axial nose includes a peripheral annular groove for receiving said first annular end portion of the gasket.
To facilitate positioning the sealing gasket and to provide good compression of its longitudinally-extending portions, at least one of the half-shells has two longitudinal grooves in its longitudinal assembly surface for receiving the longitudinally-extending portions of the gasket.
The second annular end portion of the gasket may merely be compressed axially by the second fitted end piece against the end faces of the two half-shells.
Preferably, the two half-shells comprise respective grooves in their second end faces, which grooves are shaped to receive said second annular end portion of the gasket.